FAQ
TL;DR: A rewired 28 V truck alternator driven by a 48 V / 1.5 kW e-bike controller draws just 0.6 A no-load, and “works surprisingly well” [Elektroda, Witch44, #18359775; Mnster, #17129255]. Cheap model ESCs burn above 60 A in <10 min [Elektroda, LemuRR 11, #18946374].
Why it matters: You can build a sub-€100, high-torque BLDC drive—if you avoid the common pitfalls.
Quick Facts
• Rotor resistance: 3.5 Ω → 2 A at 7 V excitation [Elektroda, LemuRR 11, #17927078]
• No-load draw: 0.6–10 A (windings & voltage dependent) [Elektroda, Witch44, #18359775; LemuRR 11, #18939300]
• Proven gear ratio: 1 : 40 moves a 1 300 kg trailer at 12 km/h [Elektroda, Witch44, post #18878781]
• 48 V 350 W sensorless controller ≈ US $15 [AliExpress Listing, 2023]
• Minimum safe controller rating: 100 A peak for ≥1 kW builds [Elektroda, Ture11, #16706658; LemuRR 11, #18946374]
1. Can a car alternator really work as a brushless DC motor?
Yes. Remove the rectifier, bring out the three stator leads, feed them with a sensorless BLDC controller, and supply separate DC to the rotor slip-rings. Users report stable operation from 12 V to 72 V systems [Elektroda, Mnster, #17129255; Witch44, #18359775].
2. Which alternator models give the best results?
Truck or industrial units rated 24–28 V and ≥90 A outperform 14 V car alternators. A Valeo 28 V 90 A unit ran icy-cold at 0.6 A no-load after rewiring, while a 14 V 150 A unit overheated above 70 % PWM [Elektroda, Witch44, post #18359775]
3. What mechanical and electrical mods are essential?
- Remove diode bridge and voltage regulator.
- Solder phase windings for star or delta as needed.
- Lead three equal wires (often blue-yellow-green) to the controller [Elektroda, Mnster, post #17789345]
Optional: convert star to delta or series-link double windings to match higher bus voltages [Elektroda, Witch44, post #18359775]
4. Which low-cost driver should I pick?
For up to 1 kW use a 48 V 20–35 A e-bike controller (≈ €15) [AliExpress Listing, 2023]. For 1–3 kW choose 48–72 V 60–100 A mobility controllers; a 1.5 kW 72 V unit survived continuous field work [Elektroda, Witch44, post #18311354] Model-aircraft ESCs run hot and fail in minutes under traction loads [Elektroda, LemuRR 11, #18946374].
5. Do I really need Hall sensors?
Not for slow starts. Several builders ran alternators sensor-less; the controller synchronised once rpm exceeded a few hundred per minute [Elektroda, Mnster, post #17789345] Halls improve stalled-torque and allow rock-crawl starts, but fitting them into narrow stator slots is difficult [Elektroda, rdt, post #18556205]
6. How much voltage and current should I feed the rotor?
Start at half the alternator’s nominal voltage. Users found 6–7 V for 14 V units and 13 V for 28 V units gave 1–3 A field current and good torque without overheating [Elektroda, LemuRR 11, #17927078; Witch44, #18363262].
7. Star or delta—what’s better?
Delta increases torque at low speed but can over-saturate the core on 48 V systems, raising no-load draw above 10 A. Re-wiring the same coils to star, or series-connecting double windings, dropped that draw to 0.6–4 A without losing pull [Elektroda, Witch44, #18359775; LemuRR 11, #18941781].
8. How do I keep controllers from burning out?
Use a controller with at least 50 % current headroom, add a heat-sink, and limit PWM to 80 % during testing. Edge-case: a cheap 24 V ESC melted its MOSFET solder at 80 A; tin literally “boiled” [Elektroda, rdt, post #17893427]
9. What gear ratio works for go-karts or scooters?
Ratios between 3 : 1 and 5 : 1 suit light scooters. Heavy karts and quads used 1 : 40 worm or chain drives to keep alternator rpm in its efficiency band and to haul >1 t loads [Elektroda, Mnster, #17895157; Witch44, #18878781].
10. How can I power the rotor from the same battery without killing buck converters?
Place an LC filter (≈ 100 µH choke + 4 700 µF electrolytic) between converter and rotor to absorb the high-frequency spikes from the stator drive, then isolate grounds with a diode. This cured four successive converter failures [Elektroda, jarek_lnx, post #18331377]
11. What happens if excitation is set too high or too low?
Too low: the controller loses sync under load, the motor stalls, and current spikes [Elektroda, Ture11, post #16706658] Too high: sudden current surge, temperature rise, and efficiency plunge—an over-saturation cliff at >70 % PWM [Elektroda, Witch44, post #18363262]
12. Quick three-step first-run procedure
- Clamp alternator securely; connect three phases to controller.
- Feed rotor 6–8 V through a 21 W bulb as a current limiter.
- Power controller at 24 V, throttle gently to check spin direction.
If smooth, raise bus voltage and field current incrementally [How-To extracted from multiple posts, #17129255 #18324834].
13. Can I run everything from a single 12 V battery?
Only for bench tests. A 14 V alternator drew 80 A at full torque, overwhelming small lead-acid batteries. Builders achieved practical ride times only above 24 V and 500 Wh capacity [Elektroda, Mnster, #16710185; Witch44, #18311354].
14. Is permanent-magnet conversion worth it?
Not usually. Fixed magnets give great stall torque but cap top speed and make field-weakening impossible. “The engine is very strong, but its speed is low,” notes an experimenter [Elektroda, Witch44, post #18878781]
15. How efficient is an alternator motor compared with a purpose-built BLDC?
Expect 60–75 % peak efficiency, about 15 percentage points below similar-power e-bike hub motors, due to laminated core losses and slip-ring friction [DOE, 2021; Elektromag* Whitepaper].
16. What wiring errors cause instant smoke?
Cross-connecting the two parallel strands of a double-wound phase in opposite polarity forms a dead short and overheats windings within seconds [Elektroda, rdt, post #18925777] Always verify phase orientation with a continuity test before powering up.
